One type of non-volatile memory is resistive memory. Resistive memory utilizes the resistance value of a memory element to store one or more bits of data. For example, a memory element programmed to have a high resistance value may represent a logic “1” and a memory element programmed to have a low resistance value may represent a logic “0”. Typically, the resistance value of the memory element is switched electrically by applying a voltage pulse or a current pulse.
One type of resistive memory is phase change memory. Phase change memory uses a phase change material in the resistive memory element. The phase change material exhibits at least two different states, including states referred to as the amorphous state and the crystalline state. The amorphous state involves a more disordered atomic structure and the crystalline state involves a more ordered lattice. The amorphous state usually exhibits higher resistivity than the crystalline state. Also, some phase change materials exhibit multiple crystalline states, e.g. a face-centered cubic (FCC) state and a hexagonal closest packing (HCP) state, which have different resistivities and may be used to store data. In the following description, the amorphous state refers to the state having the higher resistivity and the crystalline state refers to the state having the lower resistivity.
Phase changes in the phase change materials may be induced reversibly. In this way, the memory may change from the amorphous state to the crystalline state and from the crystalline state to the amorphous state in response to temperature changes. The temperature changes to the phase change material may be achieved by driving current through the phase change material itself or by driving current through a resistive heater adjacent the phase change material. With both of these methods, controllable heating of the phase change material causes controllable phase change within the phase change material.
A phase change memory including an array of memory cells that are made of phase change material may be programmed to store data utilizing the memory states of the phase change material. One way to read and write data in such a phase change memory device is to control a current and/or a voltage pulse that is applied to the phase change material. The level of current and/or voltage generally corresponds to the temperature induced within the phase change material of a memory cell.
To achieve higher density phase change memories, a phase change memory cell can store multiple bits of data. Multi-bit storage in a phase change memory cell can be achieved by programming the phase change material to have intermediate resistance values or states, where the multi-bit or multilevel phase change memory cell can be written to more than two states. If the phase change memory cell is programmed to one of three different resistance levels, 1.5 bits of data per cell can be stored. If the phase change memory cell is programmed to one of four different resistance levels, two bits of data per cell can be stored, and so on. To program a phase change memory cell to an intermediate resistance value, the amount of crystalline material coexisting with amorphous material and hence cell resistance is controlled via a suitable write strategy.
The data retention performance of a phase change memory depends strongly upon the temperature history of the memory. Typically, for nonvolatile memory, data retention is guaranteed for more than ten years at operating temperatures up to 85° C. In a phase change memory, data retention is mainly a property of the phase change material and depends on the crystallization temperature of the phase change material. For Ge2Sb2Te5, the data retention performance is about ten years at operating temperatures up to 110° C. Phase change material may begin to crystallize more quickly at temperatures greater than 150° C.
A memory device is not usually operated at a constant ambient temperature, but rather experiences significant changes in the ambient temperature. For example, a memory device for an engine controller of a car experiences extremes in temperatures based on whether the engine is running. In this situation, the data retention of a phase change memory device is impacted by the accumulated temperature budget for the memory device. In addition, data retention is more critical in multi-bit phase change memory cells than in single bit phase change memory cells.
Typically, phase change memory manufacturing includes testing and packaging and soldering, referred to as back end processing. In testing, the phase change memory die are tested and chip configuration data, such as redundancy activation data and bad block management data, is obtained via a tester. The configuration data can be stored in the phase change memory at test. Back end processing includes temperature processing of the phase change memory during packaging and soldering of the phase change memory. Typically, temperatures are 175° C. or greater during temperature processing, which may immediately affect data retention or impact the temperature budget of the phase change memory. Configuration data obtained during testing needs to be retained in the phase change memory over the life of the phase change memory device for normal operations.
For these and other reasons, there is a need for the present invention.